A. Technical Field
The present invention relates to a process for producing a water-absorbent resin of which the residual monomer content and the water-extractable content are low.
B. Background Art
Water-absorbent resins are widely utilized as absorbent articles, such as disposable diapers and sanitary napkins, by being combined with fibrous materials such as cotton, pulp, paper, and sponge.
Industrially most commonly used as the water-absorbent resins are acrylic acid-based crosslinked polymers, such as crosslinked polymers of partially neutralized poly(acrylic acids), because of their high water absorbency wherein the acrylic acid-based crosslinked polymers are obtained using acrylic acid and/or its salt as monomers (e.g. JP-A-054751/1987, JP-A-031306/1991, JP-A-211934/1994, U.S. Pat. No. 4,654,039, U.S. Pat. No. 5,338,810, U.S. Pat. No. 5,574,121, U.S. Pat. No. 5,562,646, EP 0574260 and EP 0942014).
As is mentioned above, the acrylic acid-based polymers are most commonly used because of their high water absorbency. However, their use is directed mostly to the above sanitary materials. Therefore naturally in this field the acrylic acid-based polymers are required to have no problems. The acrylic acid-based polymers are obtained by a process including the step of polymerizing at least one monomer component including acrylic acid and/or its salt as major components, and the resultant polymers contain the unreacted residue of the acrylic acid monomer although its content is very small. Therefore, the decrease of this content is required. As to the water-absorbent resin, it further needs to be water-swellable and water-insoluble, but a very small amount of water-extractable component which is a water-soluble polymer is also contained in the water-absorbent resin, and this water-extractable component has a bad influence on water absorption properties, therefore the decrease of the amount of this water-extractable component is also required. Further demanded to the water-absorbent resin are properties under a load, such as water absorption capacity under a load and liquid permeation quantity under a load.